Interconnects are generally used in integrated circuits to connect the active devices thereof. For example, in an integrated circuit such a microprocessor or a memory device containing thousands or millions of transistors, interconnects are used to connect the transistors to one another in predetermined interconnection patterns to form an operational device. As the integration density of integrated circuit devices continues to increase, multiple levels of interconnects are used to form all the requisite connections between active devices in the integrated circuit. The interconnects are generally formed of metal such as aluminum or copper.
As the integration density of integrated circuit devices continues to increase, the performance of the interconnects may have a larger impact on the performance of the integrated circuit. In particular, the interconnect structure and fabrication process may have a significant impact on the operational speed, product yields and reliability of the integrated circuit devices.
In integrated circuits with low integration density, metal interconnects are generally formed of pure aluminum. However, since aluminum can absorb silicon atoms from a silicon substrate during high temperature processing, junction spiking may occur. Accordingly, an alloy of aluminum and one percent silicon, referred to as Al-1% Si may be used instead of pure aluminum. The Al-1% Si may be obtained by oversaturating aluminum with silicon.
Unfortunately, when Al-1% Si is used for an interconnect, silicon may precipitate from the interconnect during heat treatments at about 450.degree. C. and above, thus forming silicon residues. The silicon residues may epitaxially grow in a solid phase in a contact hole, thus forming a silicon nodule. These silicon nodules may undesirably increase the contact resistance of the metal interconnect.
In order to reduce or prevent aluminum spiking, silicon residues and/or silicon nodules which are caused by reactions between a metal interconnect and a silicon substrate, it is known to use a diffusion-barrier film between the metal interconnect layer and an underlying substrate or insulation layer. For example, titanium nitride (TiN) thin films may be used as a diffusion barrier.
Copper is now being increasingly investigated and used as an interconnect for high density integrated circuits, among other reasons because copper has a lower resistivity than aluminum. Unfortunately, since copper diffuses more rapidly in a diffusion-barrier film than aluminum, a conventional TiN diffusion-barrier film may lose its ability to function as a diffusion barrier at 600.degree. C. or above. The diffusion barrier effectiveness may be lost because a TiN film may not efficiently block copper diffused along the grain boundaries which are formed in the columnar structure of a TiN film. In addition, because the TiN film is generally formed by sputtering, the TiN film may be uneven and may have poor step coverage.